Metal cargo containers are designed with provisions to secure the entry doors in order to prevent unauthorized entry into the container while it is in transit. One known approach is to secure the door handle or latch with a conventional lock. Another approach is to secure the door handle or latch with a steel bolt that is fitted with a non-removable retainer. The bolt must be cut with bolt cutters in order to release the handle or latch so that the doors can be opened.
Still another approach is to provide a device which includes a steel bolt and non-removable retainer in combination with a radio frequency identification (RFID) tag. Once the retainer has been coupled to the bolt, the RFID tag monitors the retainer and bolt and, if any tampering is detected, the tag transmits a radio signal to a remote receiver known as a reader, so that an alarm condition can be brought to the attention of a human operator and/or security personnel, who will then deal with the intrusion into the container.
Despite security measures of the type discussed above, it can be possible for the container doors to be opened without directly defeating security measures of the type discussed above. For example, it may be possible to cut the door handle or the latch. Similarly, where a door handle is fixedly connected to a rotatable connecting link which has at each end a dog that engages a recess provided in the container housing, it may be possible to cut through the connecting link and thus permit the ends of the link to be rotated so as to free the dogs from the recesses, thereby permitting the door to be opened. Still other approaches include drilling out the door handle joint, removing the door hinge pins, or cutting a hole in the sheet metal walls or roof, or through the wooden floor. There is a need for an effective technique for detecting any of these types of intrusion, in a manner that is reliable and avoids false alarms.
A further consideration is that there are some situations in which it is helpful to take into account the effects of container movement during shipment. For example, when a container is lifted from the ground and placed on a vehicle such as a truck or a ship, the position of the cargo within the container may shift. Alternatively, after the container has been loaded on a vehicle, normal vehicle movement could cause the cargo to shift position within the container. The shifting cargo should not be misinterpreted by a security system as human movement, or the security system may produce a false indication of human intrusion.
As another example, a typical cargo container is normally made of steel except for the floor, which is usually made of wood. If a monitoring system is relying on some form of electromagnetic field for the purpose of detecting unauthorized human intrusion, movement of the container may affect the electromagnetic field. For example, energy of an electromagnetic field will not readily pass through the steel walls, but will more readily pass through the wooden floor. When the container is sitting on the ground, the ground may influence characteristics such as the strength of any electromagnetic field which may be passing through the wooden floor.
If the container is then lifted off the ground, for example while it is loaded on a vehicle, the wooden floor will be spaced from the ground, thereby reducing the extent to which the ground can influence characteristics such as the strength of any electromagnetic field which may be passing through the wooden floor. To the extent a security system is relying on the electromagnetic field to try to detect human intrusion into the container, this change in the electromagnetic field as the container is lifted off the ground may be misinterpreted as human activity, and may cause the security system to produce a false indication of human intrusion. With these considerations in mind, it will be recognized that there is a need for a technique which can reliably detect unauthorized human intrusion into a cargo container, with little likelihood of false indications of intrusion as a result of factors such as container motion.